Proton electrolyte membranes (PEMs) are a critical component in fuel cells, reforming/partial oxidation of hydrocarbon fuels, hydrogen separation/purification, contaminant removal, gas sensing, and other processes relevant to energy storage and conversion. Membranes with high proton conductivity (>0.01 S/cm) but with little or no dependence on humidity in the temperature range of 100 to 200° C. are critical to a new generation of PEM fuel cells with much higher energy efficiency and tolerance of anode catalyst to carbon monoxide poisoning.
However, the conventional perfluorosulfonic-polymers, such as Nafion, suffer the serious disadvantages including poor proton conductivity in low humidity and at higher temperature range, dimensional changes in different humidity, fuel crossover, high cost and poor hydrophilicity.
Among all proton conducting membranes developed in recent years, polybenzimidazole (PBI)—H3PO4 membranes have the best performance. PBI—H3PO4 membranes have high proton conductivity (>10−2 S/cm in atmosphere with 10% relative humidity) above 150° C., good mechanical properties and high thermal stability (J Electrochem Soc 1995, Vol. 142, p. L121). However, PBI—H3PO4 membranes have been reported to have very low proton conductivity in anhydrous state (less than 1×10−4 S/cm below 160° C.) (Solid State Ion. 2002, vol. 147, p. 181 and Prog. Polym. Sci. 2000, vol. 25, p. 1463). Their proton conductivity is water dependent, too, so their application in electrochemical devices is limited. For example, they can be used as PEM electrolyte in fuel cells only when the fuel cell can produce a large amount of water in the operation. In addition, H3PO4 can leach out easily from such pure organic polymer membranes, especially when H3PO4 content is high. Also, when the content of H3PO4 is too high, the mechanical properties are degraded.
Accordingly, the development of novel electrolyte membranes with high proton conductivity in low humidity, dense structure, and good mechanical properties is still the key to the successful development of high temperature PEM fuel cells and other electrochemical devices.
Conventional materials are described in U.S. Pat. App. Pub. No. 2003/0144450, to Jacob et al., Int. Pat. App. Nos. WO01/83092 and WO01/84657 to Kerres, and U.S. Pat. No. 5,283,310 to Armand et al., U.S. Pat. No. 6,214,060 to Akita et al., and U.S. Pat. No. 6,264,857 to Kreuer et al.